I installed the software last night and got the scanner up and running. I scanned two 4x5 negatives in the Epson holder and the results were excellent. I received a reply from Betterscanning that said they don't specifically have holder for 5x7. Their method is to get a holder that's 8x10 with glass and to tape the negative to the bottom of the AN glass and use a mask, which I would have to cut, from their material to cover the negative to suit my needs. I wonder if the negative will sag even though they said to use 3M Magic tape and stretch it the best you can. The glass is above the negative so the glass acts as a heat protector.
The scanner is very nice and is easy to use. Maybe what's needed is a tiny vacuum frame.
Curt